Fully disable Gutenberg

wp-fully-disable-gutenberg-editor.php

// Fully Disable Gutenberg editor.
add_filter('use_block_editor_for_post_type', '__return_false', 10);
// Don't load Gutenberg-related stylesheets.
add_action( 'wp_enqueue_scripts', 'vietrick_remove_block_css', 100 );
function vietrick_remove_block_css() {
  wp_dequeue_style( 'wp-block-library' ); // Wordpress core
  wp_dequeue_style( 'wp-block-library-theme' ); // Wordpress core
  wp_dequeue_style( 'wc-block-style' ); // WooCommerce Block
  wp_dequeue_style( 'storefront-gutenberg-blocks' ); // Storefront theme
}